Electrochemical cells, usually classified as fuel cells or electrolysis cells, are devices used for generating current from chemical reactions, or inducing a chemical reaction using a flow of current. A fuel cell converts the chemical energy of a fuel (e.g., hydrogen, natural gas, methanol, gasoline, etc.) and an oxidant (air or oxygen) into electricity and waste products of heat and water. A basic fuel cell comprises a negatively charged anode, a positively charged cathode, and an ion-conducting material called an electrolyte.
Different fuel cell technologies utilize different electrolyte materials. A Proton Exchange Membrane (PEM) fuel cell, for example, utilizes a polymeric ion-conducting membrane as the electrolyte. In a hydrogen PEM fuel cell, hydrogen atoms are electrochemically split into electrons and protons (hydrogen ions) at the anode. The electrons flow through the circuit to the cathode and generates electricity, while the protons diffuse through the electrolyte membrane to the cathode. At the cathode, hydrogen protons combine with electrons and oxygen (supplied to the cathode) to produce water and heat.
An electrolysis cell represents a fuel cell operated in reverse. A basic electrolysis cell functions as a hydrogen generator by decomposing water into hydrogen and oxygen gases when an external electric potential is applied. The basic technology of a hydrogen fuel cell or an electrolysis cell can be applied to electrochemical hydrogen manipulation, such as, electrochemical hydrogen compression, purification, or expansion. Electrochemical hydrogen manipulation has emerged as a viable alternative to the mechanical systems traditionally used for hydrogen management. Successful commercialization of hydrogen as an energy carrier and the long-term sustainability of a “hydrogen economy” depends largely on the efficiency and cost-effectiveness of fuel cells, electrolysis cells, and other hydrogen manipulation/management systems.
In operation, a single fuel cell can generally generate about 1 volt. To obtain the desired amount of electrical power, individual fuel cells are combined to form a fuel cell stack. The fuel cells are stacked together sequentially, each cell including a cathode, a electrolyte membrane, and an anode. Each cathode/membrane/anode assembly constitutes a “membrane electrode assembly”, or “MEA”, which is typically supported on both sides by bipolar plates. Gases (hydrogen and air) are supplied to the electrodes of the MEA through channels or grooves formed in the plates, which are known as flow fields. In addition to providing mechanical support, the bipolar plates (also known as flow field plates or separator plates) physically separate individual cells in a stack while electrically connecting them. The bipolar plates also act as current collectors, provide access channels for the fuel and the oxidant to the respective electrode surfaces, and provide channels for the removal of water formed during operation of the cell. Typically, bipolar plates are made from metals, for example, stainless steel, titanium, etc., and from non-metallic electrical conductors, for example, graphite.
Additionally, a typical fuel cell stack includes manifolds and inlet ports for directing the fuel and oxidant to the anode and cathode flow fields, respectively. The stack may also include a manifold and inlet port for directing a coolant fluid to interior channels within the stack to absorb heat generated during operation of the individual cells. A fuel cell stack also includes exhaust manifolds and outlet ports for expelling the unreacted gases and the coolant water.
FIG. 1 is an exploded schematic view showing the various components of a prior art PEM fuel cell 10. As illustrated, bipolar plates 2 flank the “membrane electrode assembly” (MEA), which comprises an anode 7A, a cathode 7C, and an electrolyte membrane 8. Hydrogen atoms supplied to anode 7A are electrochemically split into electrons and protons (hydrogen ions). The electrons flow through an electric circuit to cathode 7C and generate electricity in the process, while the protons move through electrolyte membrane 8 to cathode 7C. At the cathode, protons combine with electrons and oxygen (supplied to the cathode) to produce water and heat.
Additionally, prior art PEM fuel cell 10 comprises electrically-conductive gas diffusion layers (GDLs) 5 within the cell on each side of the MEA. GDLs 5 serve as diffusion media enabling the transport of gases and liquids within the cell, provide electrical conduction between bipolar plates 2 and electrolyte membrane 8, aid in the removal of heat and process water from the cell, and in some cases, provide mechanical support to electrolyte membrane 8. GDLs 5 can comprise a woven or non-woven carbon cloth with electrodes 7A and 7C located on the sides facing the electrolyte membrane. In some cases, the electrodes 7A and 7C include an electrocatalyst material coated onto either the adjacent GDL 5 or the electrolyte membrane 8. Some high pressure or high differential pressure fuel cells use “frit”-type densely sintered metals, screen packs, expanded metals, metal foam, or three-dimensional porous metallic substrates in combination with or as a replacement for traditional GDLs to provide structural support to the MEA in combination with traditional, land-channel flow fields 4 formed in the bipolar plates 2. In some high pressure or high differential pressure cells, metal foams or three-dimensional porous metallic substrates can be used as a replacement for traditional channel-type flow fields 4 as well.
In a typical fuel cell, reactant gases on each side of the electrolyte membrane flow through the three-dimensional porous metallic flow fields or the traditional channel-type flow fields and then diffuse through the porous GDL to reach the electrolyte membrane. Since the flow field and the GDL are positioned contiguously and are coupled by the internal fluid streams, the flow field and the GDL are collectively referred to as “flow structure” hereinafter, unless specified otherwise. It is within the scope of the present disclosure to use traditional channel-type flow fields in combination with three-dimensional porous metallic GDLs, to use three-dimensional porous metallic flow fields in combination with traditional GDLs, or to use three-dimensional porous metallic substrates as both flow fields and GDLs.
Although the use of porous metallic flow structures overcome some of the physical limitations and performance penalties of high pressure or high differential pressure electrochemical cell operation, such electrochemical cells/cell stacks generally face the additional challenges of sealing the high pressure fluid within the cells and maintaining a good power-to-weight ratio. Typically, electrochemical cells, including high pressure or high differential pressure electrochemical cells, rely on separate cooling cells or cooling plates (collectively referred to as the “cooling device” hereinafter) interposed between adjacent cells in a stack. The cooling devices are generally constructed with internal fluid channels which run parallel to the horizontal plane of the stacked cells. Coolant fluid is pumped through the channels to remove heat generated during the operation of the cell stack. Heat transfer using one or more cooling devices is essential for an electrochemical cell stack with high rate of heat generation (e.g., >200 mW/cm2). However, for a cell stack operating at low heat generation rates, for example, hydrogen compressors, the separate cooling devices needlessly complicate the architecture of the cell stack, increase the cost and weight of the stack, and reduce the efficiency (i.e., decrease the electrical output) of the stack due to the added contact resistances between the cooling devices and the bipolar plates. Thus, the challenges faced by high pressure or high differential pressure electrochemical cell stacks are aggravated by convective cooling of the stacks using cooling devices between adjacent cells.
The present disclosure is directed towards the design of improved cooling systems for use in electrochemical cell stacks. In particular, the present disclosure is directed towards the design of bipolar plates for use as heat sinks (or cold plates) in conductive cooling of electrochemical cells, including, but not limited to, fuel cells, electrolysis cells, hydrogen purifiers, hydrogen expanders, and hydrogen compressors. The required cooling can be accomplished by using the one or more bipolar plates of each electrochemical cell to collect heat from the active area of the cell and to conduct the heat to at least a portion of the external boundary of the cell where the heat can be removed by traditional heat transfer means. Such an arrangement can obviate the need for using coolant fluid channels within the central, active area of the cell stack.